Swiss Pat. No. 440,134 describes a double-wall tank having an inner wall, for example made of steel, and an outer wall made of plastic. A spacer structure in form of a wire grid or wire mesh or wire weave is located between the inner wall and the outer wall. The wire mesh or grid forms a substrate for a foil forming the outer wall, located on the wire grid or wire weave. The foil described may be a cellulose foil of about 0.06 mm thickness, or an aluminum foil of about 0.1 mm thickness. The foils are used as substrates for a relatively thick layer made of glass fiber reinforced plastic, thereby forming a double-wall tank. The double wall is used to increase the safety and reliability of the tank with respect to leaks of liquid retained in the tank. A leakage warning apparatus is provided which indicates a leak at the inner wall.
The construction is difficult to make since the wire mesh as well as the foil must be handled and placed. The foil, effectively, adds only little to the strength of the wall. The wall, thus, must be made of a relatively thick plastic layer which, additionally, is reinforced with fibers, typically glass fibers. This plastic layer requires much material, and the construction thereof is relatively complex since it is applied by spraying. The spray apparatus must include a glass fiber cutter. It is unavoidable that glass fibers become loose and fly about. The glass fibers easily penetrate the skin of operators and result in itching. Loose glass fibers, additionally, interfere with working within the tank, and, in some cases, make working within the tank completely impossible.
The Swiss Pat. No. 440,134 describes such a tank in which only the outer wall is made of such plastic material. The outer wall must be carefully made to avoid the inclusion of air bubbles, requiring rolling out of the applied plastic material to eliminate the formation of any such air bubbles. Air bubbles may, otherwise, be introduced upon spraying of the plastic material together with the glass fibers. The rolling-out of the fiber-reinforced plastic wall is time-consuming, and hence expensive.
Swiss Patent of Addition No. 466,498, forming an addition to the previously discussed main patent, Swiss No. 440,134, discloses use of a plastic grid or mesh instead of a wire grid or mesh. The plastic grid has the advantage that it is substantially more flexible than metal, and is also somewhat stretchable, so that its application is simplified. In all other respects, however, the difficulties of manufacture which arise in the construction explained in connection with the main patent still pertain.
Swiss Pat. No. 471,728 describes a double-wall tank having an inner wall made of steel and an outer wall made of plastic. A spacer in the form of a foil is described, the spacer being formed with impressed or punched bumps or warts, which are facing the inner wall and are supported against the inner wall. The description proposes a spacer wall of about 0.2 mm sheet steel or sheet aluminum.
The arrangement has an advantage over the tank in accordance with the previously discussed Swiss Pat. No. 440,134 since this very thin foil can be placed about the surface of the inner wall more easily than the wire mesh which, when using a wire diameter sufficient to form a suitable chamber or hollow space between the tank walls, is comparatively stiff. All other disadvantages of the tank described in connection with Swiss Pat. No. 440,134, however, still pertain. The description is directed to the manufacture of an outer wall made of plastic, but not to an inner wall made of plastic.
Swiss Pat. No. 480,243 describes an inner jacket for a storage container which is liquid-tight. An embossed foil is located in the region of the vertical side walls. The bottom of the tank utilizes as an intermediate layer a porous mass, for example porous concrete.
Swiss Pat. No. 517,630 describes a tank to retain liquid having non-metallic walls. This liquid-containing tank has electrical conductors located on one of the wall surfaces so that the freedom from pores of the double walls of the tank can be tested; any pores which might occur can be localized by means of a spark inductor. The outer wall of the container, preferably, is made of concrete.
The conductive material is, preferably, an aluminum foil formed with bumps or warts or longitudinal ribs or ridges, for example in the form of creases. The aluminum foil may have from between 0.02 to 0.1 mm thickness. The bumps, warts, or ribs or ridges, for example longitudinal creases, form a test space for leakage between an outer wall and an inner wall. The inner wall is formed by a glass fiber reinforced plastic, for example polyester, polyurethane, or a thermoplastic material which has the required stability, pressure resistance and corrosion resistance with respect to the liquid to be introduced into the tank. The relatively thick and stiff plastic layer, and not the metallic layer, provides the necessary strength for the inner wall.
Swiss Pat. No. 618,658 describes a similar construction which is directed also to form a double bottom of an upright tank. Rather than using an aluminum foil with bumps or longitudinal creases, the description states that strong paper, such as Kraft paper or the like, or heavy wrapping paper may be used, formed with projections and recesses.
The paper is made by first applying a coating ink thereon which includes glass fibers having a fiber length of between 2.5 mm to 5 mm. The first layer applied is dried for 24 hours and then a second, usually colorless layer is applied. The strength of the inner wall is primarily governed by the layer reinforced with glass fibers; the heavy paper, formed with projections and depressions, hardly contributes anything to the strength. It is clear that, for such a construction, the plastic layer must be relatively thick. Consequently, the manufacture is time-consuming, particularly since after application of the first layer, a period of 24 hours must be permitted to elapse. The use of plastic to manufacture a thick plastic layer is extensive.